Indications for RRT in acute renal failure are as follows:
• hyperkalaemia especially if ECG changes
• volume overload
• worsening severe metabolic acidosis
• uraemic complications, for example encephalopathy, pericarditis, and seizures.
Haemodialysis removes solutes from blood by their passage across a semipermeable membrane. Heparinised blood flows in one direction and dialysis fluid flows in another at a faster rate. Dialysis fluid contains physiological levels of electrolytes except potassium, which is low, and molecules cross the membrane by simple diffusion along a concentration gradient. Smaller molecules move faster than larger ones. Urea and creatinine concentrations are zero in the dialysis fluid because they are to be removed as much as possible. A 3-4-hour treatment can reduce urea by 70%. Water can be removed by applying a pressure gradient across the membrane if needed.
Haemofiltration involves blood under pressure moving down one side of a semipermeable membrane. This has a similar effect to glomerular filtration, and small and large molecules are cleared at the same rates. Instead of selective reabsorption, which occurs in the kidney, the whole filtrate is discarded and the patient is infused with a replacement physiological solution instead (Box 8.4). Less fluid may be replaced than is removed in cases of fluid overload. In original haemofiltration, the femoral artery and vein were cannulated (continuous arteriovenous haemofiltration or CAVH). Blood
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